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Register a new userDecelerated non-relativistic expansion in a tidal disruption event with a potential neutrino association
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A tidal disruption event (TDE) involves the tidal shredding of a star in the vicinity of a dormant supermassive black hole. The nearby ($approx$230 mega-parsec) radio-quiet (radio luminosity of $4 times 10^{38}$ erg s$^{-1}$) AT2019dsg is the first TDE potentially associated with a neutrino event. The origin of the non-thermal emission in AT2019dsg remains inconclusive; possibilities include a relativistic jet or a sub-relativistic outflow. Distinguishing between them can address neutrino production mechanisms. High resolution very long baseline interferometry monitoring provides uniquely constraining flux densities and proper motion of the ejecta. A non-relativistic (outflow velocity of $approx$0.1 $c$) decelerated expansion in a relatively dense environment is found to produce the radio emission. Neutrino production may be related to the acceleration of protons by the outflow. The present study thus helps exclude jet-related origins for the non-thermal emission and neutrino production, and constrains non-jetted scenarios.
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Cosmic neutrinos provide a unique window into the otherwise-hidden mechanism of particle acceleration in astrophysical objects. A flux of high-energy neutrinos was discovered in 2013, and the IceCube Collaboration recently associated one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the association of a radio-emitting tidal disruption event (AT2019dsg) with another high-energy neutrino, identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility (ZTF). The probability of finding any radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multi-zone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for PeV neutrino production. The association suggests that tidal disruption events contribute to the cosmic neutrino flux. Unlike previous work which considered the rare subset of tidal disruption events with relativistic jets, our observations of AT2019dsg suggest an empirical model with a mildly-relativistic outflow.
We present the discovery of PS18kh, a tidal disruption event (TDE) discovered at the center of SDSS J075654.53+341543.6 ($dsimeq322$ Mpc) by the Pan-STARRS Survey for Transients. Our dataset includes pre-discovery survey data from Pan-STARRS, the All-Sky Automated Survey for Supernovae (ASAS-SN), and the Asteroid Terrestrial-impact Last Alert System (ATLAS) as well as high-cadence, multi-wavelength follow-up data from ground-based telescopes and Swift, spanning from 56 days before peak light until 75 days after. The optical/UV emission from PS18kh is well-fit as a blackbody with temperatures ranging from $Tsimeq12000$ K to $Tsimeq25000$ K and it peaked at a luminosity of $Lsimeq8.8times10^{43}$ ergs s$^{-1}$. PS18kh radiated $E=(3.45pm0.22)times10^{50}$ ergs over the period of observation, with $(1.42pm0.20)times10^{50}$ ergs being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked H$alpha$ emission feature, which becomes more prominent over time. We use models of non-axisymmetric accretion disks to describe the profile of the H$alpha$ line and its evolution. We find that at early times the high accretion rate leads the disk to emit a wind which modifies the shape of the line profile and makes it bell-shaped. At late times, the wind becomes optically thin, allowing the non-axisymmetric perturbations to show up in the line profile. The line-emitting portion of the disk extends from $r_{rm in}sim60r_{rm g}$ to an outer radius of $r_{rm out}sim1400r_{rm g}$ and the perturbations can be represented either as an eccentricity in the outer rings of the disk or as a spiral arm in the inner disk.
We present continued radio and X-ray observations of the previously relativistic tidal disruption event (TDE) Swift J164449.3+573451 (sw) extending to about 9.4 years post disruption, as part of ongoing campaigns with the Jansky Very Large Array (VLA) and the textit{Chandra} X-ray observatory. We find that the X-ray emission has faded below detectable levels, with an upper limit of $lesssim 3.5times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ in a 100 ks observation, while the radio emission continues to be detected and steadily fade. Both are consistent with forward shock emission from a non-relativistic outflow, although we find that the radio spectral energy distribution is better fit at these late times with an electron power law index of $papprox 3$ (as opposed to $papprox 2.5$ at earlier times). With the revised spectral index we find $epsilon_Bapprox 0.01$ using the radio and X-ray data, and a density of $approx 0.04$ cm$^{3}$ at a radius of $Rapprox 0.65$ pc ($R_{rm sch}approx 2times 10^6$ R$_odot$) from the black hole. The energy scale of the blastwave is $approx 10^{52}$ erg. We also report detections of sw at 3 GHz from the first two epochs of the VLA Sky Survey (VLASS), and find that $sim 10^2$ off-axis sw-like events to $zsim 0.5$ may be present in the VLASS data. Finally, we find that sw itself will remain detectable for decades at radio frequencies, although observations at sub-GHz frequencies will become increasingly important to characterize its dynamical evolution.
A small fraction of candidate tidal disruption events (TDEs) show evidence of powerful relativistic jets, which are particularly pronounced at radio wavelengths, and likely contribute non-thermal emission at a wide range of wavelengths. A non-thermal emission component can be diagnosed using linear polarimetry, even when the total received light is dominated by emission from an accretion disk or disk outflow. In this paper we present Very Large Telescope (VLT) measurements of the linear polarisation of the optical light of jetted TDE Swift J2058+0516. This is the second jetted TDE studied in this manner, after Swift J1644+57. We find evidence of non-zero optical linear polarisation, P_V ~ 8%, a level very similar to the near-infrared polarimetry of Swift J1644+57. These detections provide an independent test of the emission mechanisms of the multiwavelength emission of jetted tidal disruption events.
Aims. We investigate the evolution of X-ray selected tidal disruption events. Methods. New events are found in near-real time data from XMM-Newton slews and are monitored by multi-wavelength facilities. Results. In August 2016, X-ray emission was detected from the galaxy XMMSL2 J144605.0+685735 (a.k.a. 2MASX 14460522+6857311), a factor 20 times higher than an upper limit from 25 years earlier. The X-ray flux was flat for ~100 days and then fell by a factor 100 over the following 500 days. The UV flux was stable for the first 400 days before fading by a magnitude, while the optical (U,B,V bands) have been roughly constant for 850 days. Optically, the galaxy appears to be quiescent, at a distance of $127pm{4}$ Mpc (z=$0.029pm{0.001}$) with a spectrum consisting of a young stellar population of age 1-5 Gyr, an older population and a total stellar mass of ~6 x $10^{9}$ solar masses. The bolometric luminosity peaked at L bol ~ $10^{43}$ ergs s$^{-1}$ with an X-ray spectrum that may be modeled by a power-law of $Gamma$~2.6 or Comptonisation of a low-temperature thermal component by thermal electrons. We consider a tidal disruption event to be the most likely cause of the flare. Radio emission was absent in this event down to < 10$mu$Jy, which limits the total energy of a hypothetical off-axis jet to E < 5 x $10^{50}$ ergs. The independent behaviour of the optical, UV and X-ray light curves challenges models where the UV emission is produced by reprocessing of thermal nuclear emission or by stream-stream collisions. We suggest that the observed UV emission may have been produced from a truncated accretion disk and the X-rays from Compton upscattering of these disk photons.
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